When handling transmission streams in PDH at 1544 kb/s, it is accepted to arrange them in multiframes. Multiframes are used, for example, to transmit a number of telepnone conversations via a common data channel, wherein each frame in the multiframe carries information relevant to a different telephone call. In PDH, there are two alternative basic multiframe structures: 24-frame structure and a 12-frame one, respectively comprising 24 or 12 standard PDH data frames. It is understood, that if information is transmitted in long data blocks such as multiframes structures, there is a need to define the beginning and the end of such a structure, in other words—a need to synchronize the data so as to calculate positions of any characteristic bytes and bits in the frames. Each of these standard multiframe streams comprises a so-called FAS portion (bits forming a Frame Alignment Signal) which constitutes a binary signature. The principle of using FAS in multiframes is explained in the Standard Recommendation G.706 (1991). In the 24-frame multiframe structure, one bit of those assigned to bear the FAS is placed in a particular position of every fourth frame, so that six such bits form in succession a FAS code “001011” (so-called periodic signature). This widely used arrangement was proposed in an ITU-T Standard Recommendation G.704 (7/95), namely, in table I/G.704 for the 24-frame multiframe structure.
The Standard Recommendation G.704 speaks about the Multiframe alignment signal (FAS) as follows: “The F-bit of every fourth frame forms the pattern 001011 . . . 001011. This multiframe alignment signal is used to identify where each particular frame is located within the multiframe in order to extract the cyclic redundancy check code, CRC-6, and the data link information, as well as to identify those frames that contain signaling (frames 6,12,18,24), if channel associated signaling is used.” It means, that one F-bit belonging to FAS can be found in every four frames.
Position of the FAS code in the 12-frame PDH structure, and the signature itself differ from those in the 24-frame structure; they are defined in Table 10/G.704 of the same G.704 document. It should be noted, however, that the principle of inserting periodic signatures is similar also for other multiframe structures, such as those used in PDH systems.
If the proper place of a periodic signature in the multiframe is known, and if presence of the periodic signature is detected in a currently arriving data stream, various required operations and control functions related to the data stream can be synchronized with respect to the determined position of the signature. In prior art, the signature position (once determined in the current data stream) is usually re-checked for confirmation, by obtaining the same signature “n” times with the known periodicity.
However, the above-mentioned standards, as well as other standards known to the Applicant, do not explain how the periodic signatures can be revealed in the data stream. Existing techniques of doing that are quite cumbersome and require a high expenditure of hardware. For example, a great number of interacting final state machines (FSM) can be designed to catch a cyclic signature in a multiframe.